Computer for preparation of color separations



June 6, 1967 D. J. KYTE 3,324,235

CO PUTER FOR PREPARATION OF COLOR SEPARATIONS Filed NOV- 6, 1964 CORRECTED MAGENTA OUTPUT Dm \MAGENTA INPUT ADD sua- TRACT Dc \CYAN INPUT\ 0 PRIMARY *conascnou M SIGNAL 1 SUB- TRACT DY YELLOW INPUT 2 .SlGNAL 2 DEREK J m KYTE United States Patent 3,324,235 COMPUTER FOR PREPARATION OF COLOR SEPARATIONS Derek John Kyte, 117 Valley Road, Chorleywood, England Filed Nov. 6, 1964, Ser. No. 409,448 Claims priority, application Great Britain, Nov. 8, 1963, 44,261/ 63 5 Claims. (Cl. 178--5.2)

This invention relates to colour printing and more particularly to methods of, and means for, the preparation of a plurality of so-called photographic colour separation plates or films or the like from a coloured original, each of which is in monochrome but in each of which the densities of the silver image purport to represent the amounts of corresponding coloured inks, for example yellow, magenta, cyan and black, which are to be superimposed to give a printed reproduction of the coloured original.

The object of the invention is to obviate difficulties previously encountered in the preparation of such plates.

The main aspect of the invention comprises electrical colour correction equipment for use in colour printing wherein electrical colour separation signals are generated in response to photo-electric scanning of a coloured picture through colour filters, and which comprises means for correcting each separation signal on a basis which is different according as the separation signal to be corrected is of greater or less amplitude than the correcting separation signal.

Each separation signal train can act as a primary correction signal for some or all of the other separation signal trains involved in a colour separation system, and the secondary correction signal train derived from the interaction of a separation signal train and the respective primary correction signal trains, can then be used to modify the original colour separation signal train to give a realistic result.

The derivation of each secondary correction signal train can involve manually-adjustable means for regulating the operations according to the characteristics of the inks involved.

It is further possible to introduce a threshold value for the secondary correction signals in order to improve the signal-to-noise ratio.

Patented June 6, 1967 Published theoretical work in this field has shown that in general, a correct colour separation can only be produced by combining two or more records of the coloured original, these records being prepared by photographingor otherwise recording-the original through different coloured filters.

It is possible from such theoretical considerations to deduce equations which show how the transmission density of the separation is related to the transmission or reflection densities of the original measured with various filters. The simplest equations are the so-called continuous tone masking equations. Such an equation for a yellow separation takes the form:

Where The invention will be described with reference to an embodiment shown in the accompanying drawing which shows diagrammatically an electronic computing circuit for automatically adjusting a colour separation in accordance with the invention. It will be understood that one such circuit will be provided for each colour separation.

In any of the processes of making multi-colour printing plates from a coloured original, it is at some stage necessary to prepare so-called colour separations. These may be photographic plates or films where the density of the developed silver image represents the amount of ink of a particular colour which-in combination with certain other coloured inks-will give a satisfactory printed reproduction of the original. For example, in a so-called Yellow separation for 3 or 4-colour printing the density 7 of the silver image should represent the amount of yellow ink which, together with certain amounts of magenta, cyan and black, will give a reasonable printed reproduction of the original.

Because available inks do not conform to the theoretical D is the required density on the yellow colour separation,

D D and D are the densities of the original measured through blue, green and red filters respectively,

*y is a constant associated with the requirements of the printing process and a and B are masking co-eflicients associated with the particular printing inks to be used.

These equations can be wholly or partly solved by rather lengthy photographic processes. They can also be solved mathematically, by electronic computers, for example, the input information being provided in these cases by photoelectric scanning techniques.

The solution of the type of equation quoted above will give colour separations which are not completely correct-even for continuous tone processesalthough they may often be acceptable. The reason for this is that the equations themselves are based on assumptions which are not strictly true. One of these assumptions is that printing inks are completely transparent. Another is the principle of additivity of densities, that when ink layers are superimposed, the density of the combination is equal to the sum of the densities of the individual layers.

It, as is usually the case, these assumptions are not true, the effect is that the colour separations produced according to equations of the above kind are correct in areas where only a single ink is printed, but incorrect where two or more inks are superimposed. Typically, the effect on the printed reproduction will be that greens will contain too little yellow, orange-reds will contain both too little magenta and too little yellow, violets will contain too little magenta and all dark colours including greys and blacks will lack both yellow and magenta.

One method of correcting these faults is to expand the density range of the corrected colour separation so that the amount of ink printed in black areas is sufficient. Methods exist in both photographic and mathematical techniques for achieving this expansion. The result in both cases, however, is that although the blacks and greys are improved, the other mixture colours are not improved to the same extent. Typically, the effect of such expansion is that the density of the yellow separation is too high in yellow and the density of the magenta separation is too high in orange-reds.

I The accompanying drawing shows an example of a computer circuit automatically adjusting col-our separations in a manner which gives more accurate and acceptable results than known techniques of all kinds. The circuit shown is for a magenta separation; similar circuits would be provided for the yellow and cyan separations but not, of course, for the' black. The circuit comprises subtracting adding and discrimination circuits of any desired type according to the nature of the signal in use. The diodes D1-D4 form discriminating circuits 3 suitable for use with DC signals. For AC signals, these diodes would be replaced with suitable phase discrimination circuits.

In this figure, thethree input signals D D and D are preferably proportional to the densities of the original picture through green, red and blue filters respectively. By density in this context is meant a logarithmic function of the transmittance or reflectance of the original picture. Trains of these input signals may be derived by scanning the original picture by photo-electric means, i.e., by illuminating it with a suitable light source and picking up the light reflected or transmitted from each successive small element of the picture by three photo-cells, each viewing the light through a different colour filter. Alternatively, the input signals; may be derived from the simultaneous scanning of three uncorrected photographic separations, or by any other convenient means. The input signal D which may be referred to as the uncorrected magenta signal, has the uncorrected cyan input signal D (the primary correction signal) subtracted from it in the circuit marked Subtract 1. The result of the subtraction is referred to as a secondary correction signal and according to whether it is positive or negative, passes either through diode D1 and Potentiometer P1 or diode D2 and Potentiometer P2. In either case, the signal is added to the original magenta input signal D in the circuit Add 1.

Similarly, the input signal D has subtracted from it the yellow input signal D in circuit Subtract 2. The resultant signal is fed via diode D and Potentiometer P3 or diode D4 and Potentiometer P4 to the circuit Add 1 where it is added to the input signal. The way in which this circuit operates may be seen by considering what happens when the input signals corespond to the scanning of certain colours in the original.

The table below shows typical values of the input signals M D and D obtained when scanning eight colours on the original picture. These colours are three subtractive primaries yellow, magenta and cyan (supposed in this instance to be matched by the three printing inks to be used), the three first order mixture colours obtained by taking the primaries in pairsorange/red, violet and green, and the triple mixture black.

SCANNED DENSITY Colour E". N ocowo 0 www- All colours (except white) are assumed to he of maximum saturation possible, i.e., violet has to be reproduced by full cyan plus magenta plus zero yellow, etc. On a positive photographic colour separation, the density corresponding to white in the original may be regarded as an instruction PRINT NO INK whilst the density corresponding to black on the original may be considered as meaning PRINT FULL INK. A separation made directly from the scanned density D in the above chart would be faulty in every colour except white and black. In particular, it would indicate that magenta ink should be printed in yellow, cyan and green, whereas in fact these colours require zero magenta. Also, it would indicate that less than full magenta should be printed in orange-red and magenta.

The greatest density found on a colour transparency is normally that of Black. If this density is taken as a lower reference point, that is, as the Print full ink point, then the function of the computer will be to lower the densities of White colours and to increase the densities of Black colours.

Alternatively, the density of the separation colour e.g., Magenta (1.56 in the example) can be taken as the lower reference point. In this case, the function of the computer will be to lower the densities of white colours as before and also to lower the densities of black colours and black to the density of 1.56. The final result will still be the same although the chosen end densities are different.

In practice, this latter approach is not very desirable because the density of the separation colour is not the same on each separation. (Magenta: 1.56 on the magenta separation, Yellomlfio on the yellow separation and Cyan=1.93 on the cyan separation). Thus the lower reference density would be different for each separation. If, however, we always choose black as a lower reference point, this density is identical on all separations.

In order to give a correct separation, therefore, it is necessary that D should be reduced in the so-called White coloursyellow, cyan and green, and increased in the so-called Black colours: orange-red and magenta.

In photographic and electronic col-our correction processes, it is normal to concentrate on correcting the White colours because faults here result in dirty reproductions and are most noticeable to the human eye. The elfect of the correction process on the black colours is usually taken as it comes, although certain compromise methods are occasionally used. If the additivity law holds and inks are transparent the correction of black colours will be correct automatically if the white colours are correct. However, as stated earlier, this is not generally true.

The purpose of the circuit described here is to separate the correction process for the white colours and the black colours so that independent adjustment is possible for each. In this way, better colour separations can be produced.

Referring now to the drawing, all input signals are equal when scanning black or grey tones. Thus the result of the subtractions in circuits Subtract 1 and Subtract 2 is zero and the magenta output signal Dm is equal to the magenta input signal D When scanning cyan, which can only be produced by cyan ink and requires no magenta, the input signal D is too high. However, D is much higher so the output of Subtract 1 will be a negative signal. This signal passes through diode D2 and potentiometer P2 and is added to the input signal D in Add 1. When P2 is set correctly, the result of this addition is zero i.e. the corrected magenta signal when scanning cyan is zero and no magenta ink will be printed in areas which represent cyan on the original.

When scanning orange-red, the uncorrected magenta signal is not high enough since this colour requires to be reproduced with full magenta (and full yellow). In this case, the cyan input signal is low so that the result of the subtraction in Subtract 1 is a positive signal which passes through diode D1 and potentiometer P1. If the latter is set correctly, the result of adding this signal to the input signal D will be to increase the output signal to the Value representing black, i.e. full printing density.

Similarly, when scanning yellow, the output of Subtract 2 is negative and potentiometer P4 is adjusted so that the output signal D is zero. When scanning violet, the output of Subtract 2 is positive and the potentiometer P3 is adjusted so that the output signal D represents the full printing level.

The advantages of this circuit are that:

(a) The grey scale is not affected atall by the correction process.

('b) The separation of the positive and negative outputs from the Subtract l and Subtract 2 circuits enables the colour correction to be separately controlled for White colours and for Black colours.

In practice, judicious adjustment of the four controls enables a colour separation to be produced which is as near perfect as could be desired.

The circuits for the yellow and cyan separations are similar to that described for magenta, except that the colour separation signal to be adjusted is in each case connected to Subtract l and to Add 1, the other two colour separation signals in each case being connected one to Subtract 1 only and the other to Subtract 2 only.

The diodes D1 and D4 may be valves or semi-conductor diodes. In practice, it is convenient to use semi-conductors since they ofier a high impedance until the outputs of the Subtract l and Subtract 2 circuits are greater than about 0.5 volts. When scanning deep blacks, where the signal/ noise ratio is the lowest, these diodes elfectively prevent the noise of the secondary correction signal from being added to the separation signal in Add 1.

The above description refers to a circuit in which D.C. signals are used. Exactly the same principles can be applied to AC. signals, although in this case the diodes D1-D4 have to be replaced by phase discrimination networks.

In the electronic computing process for the production of colour separations described above the circuits for producing colour correction have no efiect on neutral tones, this being achieved by forming the correction signal by subtracting from the separation input signal other incoming signal or signals representing records of the originals through other filters.

The correction of colours requiring none of or little of the separation colour can be adjusted separately from the correction of colours requiring full or large amounts of the separation colour, this being achieved by passing the secondary correction signals referred to above through discriminating circuits which alter the amplitude of the signal by different adjustable amounts according to whether the magnitude of the separation signal is greater or less than the magnitude of the primary correction signal.

The outputs from the two attenuating devices are then added to the original separation signal.

The signal/noise ratio when scanning very dark greys is improved by the incorporation of a threshold value in the discriminating device so that its outputs are substantially zero until the input exceeds a certain magnitude.

The primary correction signals may be changed from positive to negative sign, or to the opposite phase, and added to the separation signal in Adding circuits replacing Subtract 1 and Subtract 2.

What I claim is:

1. Electrical color correction equipment for use in color printing wherein electrical color separation signals are generated in response to photo-electric scanning of a colored picture through color filters, and which comprises a separate electrical correction means for each separation signal, each of said correction means comprising a set of electrical comparison means for comparing first and second applied signals and producing an output signal having a first characteristic or a second characteristic according as the amplitude of said first signal is greater or less than the amplitude of said sec-0nd signal, respectively, means for applying as the first signal to each comparison means the separation signal to be corrected, means for applying as the second signal to each comparison means a different one of the other separation signals, a pair of independently adjustable electrical correction signal generating means for generating two independent correction signals associated with each of said comparison means, signal combining means for producing an output signal in accordance with the aggregate of a set of applied signals, means for applying the associated separation signal to said signal combining means, and means controlled by each comparison means for applying the correction signal from the first or the second associated correction signal generating means to the signal combining means according as the output signal from the comparison means has said first or said second characteristic, respectively.

2. The apparatus of claim 1, in which said combining means comprises summing means for producing an output signal in accordance with the algebraic sum of a set of applied signals, and in which each comparison means and its associated pair of signal generating means comprises means for applying to said summing means a correction signal having an amplitude proportional to the absolute magnitude of the diiference between the signals applied to the comparison means by a factor of proportionality determined by the extent of adjustment of the selected signal generating means and a sign determined by the sign of said difference.

3. The apparatus of claim 2, in which each comparison means and its associated pair of signal generating means comprises signal subtraction means responsive to the applied separation signals for producing an output diiference signal having a sign and magnitude in accordance With the difierence between the applied signals, first and second gate means connected to said subtraction means, said first gate means producing an output signal in accordance with the difiference signal from said subtraction means when said dilference signal is positive and said second gate means producing an output signal in accordance with the difference signal when the difference signal is negative, adjustable attenuating means controlled by each gate means and responsive to an output signal from the gate means for producing a correction signal equal to the output signal from the gate means attenuated by a factor dependent on the extent of adjustment of the attenuating means, and means for applying said correction signal to said summing means.

4. The color correction equipment of claim 1, further comprising means for adding a fixed threshold attenuation signal to each correction signal to improve the signal-tonoise ratio.

5. The color correction equipment of claim 3, further comprising means for adding a fixed threshold attenuation signal to each correction signal to improve the signal-tonoise ratio. 

1. ELECTRICAL COLOR CORRECTION EQUIPMENT FOR USE IN COLOR PRINTING WHEREIN ELECTRICAL COLOR SEPARATION SIGNALS ARE GENERATED IN RESPONSE TO PHOTO-ELECTRIC SCANNING OF A COLORED PICTURE THROUGH COLOR FILTERS, AND WHICH COMPRISES A SEPARATE ELECTRICAL CORRECTION MEANS COMPRISING A SET OF SIGNAL, EACH OF SAID CORRECTION MEANS COMPRISING A SET OF ELECTRICAL COMPARISON MEANS FOR COMPARING FIRST AND SECOND APPLIED SIGNALS AND PRODUCING AN OUTPUT SIGNAL HAVING A FIRST CHARACTERISTIC OR A SECOND CHARACTERISTIC ACCORDING AS THE AMPLITUDE OF SAID FIRST SIGNAL IS GREATER OR LESS THAN THE AMPLITUDE OF SAID SECOND SIGNAL, RESPECTIVELY, MEANS FOR APPLYING AS THE FIRST SIGNAL TO EACH COMPARISON MEANS THE SEPARATION SIGNAL TO BE CORRECTED, MEANS FOR APPLYING AS THE SECOND SIGNAL TO EACH COMPARISON MEANS A DIFFERENT ONE OF THE OTHER SEPARATION SIGNALS, A PAIR OF INDEPENDENTLY ADJUSTABLE ELECTRICAL CORRECTION SIGNAL GENERATING MEANS FOR GENERATING TWO INDEPENDENT CORRECTION SIGNALS ASSOCIATED WITH EACH OF SAID COMPARISON MEANS, SIGNAL COMBINING MEANS FOR PRODUCING AN OUTPUT SIGNAL IN ACCORDANCE WITH THE AGGREGATE OF A SET OF APPLIED SIGNALS, MEANS FOR APPLYING THE ASSOCIATED SEPARATION SIGNAL TO SAID SIGNAL COMBINING MEANS, AND MEANS CONTROLLED BY EACH COMPARISON MEANS FOR APPLYING THE CORRECTION SIGNAL FROM THE FIRST OR THE SECOND ASSOCIATED CORRECTION SIGNAL GENERATING MEANS TO THE SIGNAL COMBINING MEANS ACCORDING AS THE OUTPUT SIGNAL FROM THE COMPARISON MEANS HAS SAID FIRST OR SAID SECOND CHARACTERISTIC, RESPECTIVELY. 